420 4.2.0 Transient Failure during recipients lookup AmbiguousRecipientTransientException

Hello **** Vince DiNenna

Good day and thanks for reaching out.

It sounds like you’re dealing with a complex issue where emails are failing to be delivered to certain external recipients, and you've already taken several troubleshooting steps, including running extended trace reports and working with Microsoft support.

The issue you are describing "420 4.2.0 Transient Failure during recipient lookup" and "AmbiguousRecipientTransientException" indicates that there is likely a deeper issue with recipient resolution within Exchange Online that goes beyond common configuration problems. On the other hand, the 420 4.2.0 Transient Failure and 550 4.4.7 QUEUE.Expired errors are indicative of emails timing out due to server resolution failures. This could be caused by temporary issues between Exchange Online and the recipient’s email server.

Given that this involves how emails are categorized, resolved, and routed internally within Exchange Online, it’s something that Microsoft’s backend Exchange Online engineers will need to investigate further. These types of resolution issues can sometimes require adjustments to your tenant's Exchange Online settings or a deeper analysis of how your mail flow is being processed. Recipient resolution in Exchange Server | Microsoft Learn

Microsoft’s Exchange Online backend can have difficulties determining which recipient is the correct one to route the email to, especially in environments with hybrid or mixed tenant scenarios (internal users and external guest users). These types of errors often require backend system-level diagnostics that only Microsoft engineers can perform. If the emails are failing consistently for specific recipients but not others, it’s essential to trace the failure points through the backend mail flow logs to pinpoint where the breakdown occurs.

Since you have already worked with Microsoft support, but the issue remains unresolved, it’s advisable to escalate the ticket. Can you please share the ticket number with us so we can follow up or escalate the issue to senior resources if necessary.

Kindly share the support ticket number with us, and feel free to share any other details you think might help us resolve the issue. I look forward to your reply and am here to assist further. Please understand that our initial response may not always resolve the issue immediately. However, with your help and more detailed information, we can work together to find a solution.

Thank you for your valuable time and cooperation.

Sincerely,

Bertrand | Microsoft Community Moderator

Hello Bertrand,

Thank you for your reply. The MS ticket number is #‎241******.

I ran another extended mail trace last night but this morning the reports will not download. Will run again if MS support requests a report.

Thank you!

Vince

Hello Vince DiNenna

Good day and thanks for reaching out.

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Sincerely,

Bertrand | Microsoft Community Moderator

I have the same exact issue. Identical symptoms. Have not found a solution yet, working with a technician. Ours started Wed on the 18th? (then wasn't happening for a few hours yesterday afternoon) back again a few hours later.

One thing I found: If you went to list guest users in the main admin panel, the users with the issue were the only people who's "Display Name" was set as their email address.

Changed that to a first name, but this did not fix the issue. Also deleted guest user from Azure Active Directory Guests, also did not fix

We also started experiencing the same issues last week. MS Support is still working on the issue. I thought it might be guest users as well, so I deleted the accounts in question, but it has not resolved the issue.